Document wxWebResponse::GetContentLength()

Also change its return type from wxInt64 to wxFileOffset for consistency
with all the other length/progress-related functions in wxWebRequest.

interface/wx/webrequest.h

@@ -394,6 +394,8 @@ public:
 
         This value is based on the @c Content-Length header, if none is found
         it will return -1.
+
+        @see wxWebResponse::GetContentLength()
     */
     wxFileOffset GetBytesExpectedToReceive() const;
     ///@}
@@ -515,6 +517,14 @@ public:
     */
     wxString GetHeader(const wxString& name) const;
 
+    /**
+        Get the length of returned data if available.
+
+        Returns the value specified in the @c Content-Length: response header
+        of @c -1 if not available.
+     */
+    wxFileOffset GetContentLength() const;
+
     /**
         Returns the MIME type of the response (if available).
     */